Why am I an atheist? Short answer: I was born that way, and never saw a reason to change.
Slightly longer answer: while in my youth I got some of the trappings of what I’d guess would be called the Lutheran faith, such as baptism and Confirmation, mostly (as I understand it) for reasons external to my family (such as convincing Christian adoption agencies to let my parents adopt three more children), religion pretty much didn’t happen in my family. In fact, given that I remember getting final confirmation of my distrust in Santa Claus, I probably believed in him for a while, which is certainly more than I can say about any god.
I do remember when I first started reading the Bible, it seemed a familiar kind of text: pretty much like other mythological texts I already knew, such as the Nibelungen. So the idea in “believing” in this stuff was always rather weird.
Fortunately, while it’s rather hard to escape religion as a school subject over here, the protestant variant at least for me never included any checks of or requests for belief that I can remember. It was perfectly fine to take that stuff as an atheist (which I can’t say for the – I expect in the US it’s called “Sunday school” – leading up to the confirmation, which gave me serious ethical stomach aches: I don’t like to lie about what I believe).
Not until I first learned about them on Usenet (in sci.sceptic, I believe, back when that was still readable, and a certain VAX (or was that VMS? The memory is fading alarmingly) guy was almost my hero) did I dream of the possibility of Creationists. (And the first people I told of them plainly didn’t believe me.) I believe that’s about the same time I learned that the correct term for my (non)belief was atheism.